1. Technical Field
The present invention relates, in general, to a method and apparatus for automatically profiling surfaces and, in particular, to feature measurements to sub-nanometer region using atomic force microscopy. More specifically still, the present invention is directed to surface profile analysis for submicron structures that assesses the height, width, and side wall angle of a device under test.
2. Description of the Related Art
The capability to measure line widths and profile trenches and substrates is becoming more and more important in the field of micro-meterology. Presently, instruments based on optical interaction are inaccurate or have physical limitations when the features to be measured are smaller that a micrometer.
Accurate micro-meterology is typically performed with scanning electron microscopes (SEM). There are several disadvantages to the use of a SEM, such as the need to perform measurements in a vacuum environment and cross sectioning of the substraights where the measurements are performed at only one location of the grove or trench. SEM measurement is a time consuming process and there is a limited spacial accuracy due to the effects resulting from e-beam interaction with the material being measured.
The use of scanning tunneling microscopes to measure forces between a tip and a surface of an insulating material by simply mounting a scanning tunneling microscope tip on a cantilever beam is another system used to perform submicron analysis. The resulting instrument combines the principles of the scanning tunneling microscope and the stylus profilometer.
Next, atomic force microscope mapping and profiling has also been developed. An atomic force microscope is used to provide precise measurements of the force between a tip and a sample over a tip sample distance in the range between approximately 30-50 Angstroms. In a first application, the force signal is used to maintain constant tip-surface spacing for facilitating profiling with a spacial resolution of 50 Angstroms.
In the prior art techniques, the top edges of the trench are detected by first vertically moving the tip toward the top surface of the trench and then scanning the tip horizontally within the trench. The trench measurements are then performed by lowering the tip in the center of the trench. The tip is made to approach horizontally one of the side walls and then the other in order to measure the width at a specific depth location. The depth of the tip in the trench is changed and the two side wall approach technique is repeated. At each depth location, two measurements are performed, as the tip approaches the first and the second side wall, which is time consuming. Moreover, a complex dual optical sensor is required for first performing the vertical and then the horizontal approaches.
In prior apparatus, the tip is vibrated in the horizontal direction as well as the conventional vertical direction, possibly at difference frequencies, in order to sense the horizontal and vertical components of the force gradient. A serious limitation of such an arrangement is the need to measure the horizontal vibration. However, by using a second interferometer with its light beam focused on the light beam of the cantilever supporting the tip, the horizontal vibration may be measured. Such an arrangement significantly increases the complexity of the apparatus.
With the vibration of the tip, a certain level of noise is introduced into the measurement signal inadvertently. Additionally, the systems must be manually operated by a user to profile each trench one by one to complete the analysis.
Accordingly, what is needed is a system and method for profiling submicron structures that greatly minimizes spurious noise in the measurement signals while also allowing automatic profiling of many trenches on a surface, rather that manually profiling one trench at a time.